In the male the testicles pass out of the ventral cavity, and penetrate by the inguinal canal into a sac-shaped fold of the outer skin. The various mammals bring before us the successive stages of this displacement. In the elephant and the whale the testicles descend very little, and remain underneath the kidneys. In many of the rodents and carnassia they enter the inguinal canal.
The phylogeny of the placenta has become more intelligible from the fact that we have found a number of transitional forms of it. The four great legions of the Placentals Rodents, Ungulates, Carnassia, and Primates are sharply separated to-day by important features of organisation.